The present invention relates to a diagnostic method and a diagnostic system for a multicylinder internal combustion engine that determines the occurrence of a malfunction pertaining to an air-fuel ratio in a cylinder based on rotational fluctuations in the engine during lean-burn operation.
Recently, in a given operating area of an internal combustion engine using gasoline fuel, for example, in an idle operation state, it has been proposed that engine operation concerning the air-fuel ratio be leaner than the stoichiometric air-flow ratio, i.e., lean-burn operation is conducted to increase gas mileage of the engine. The lean-burn operation is achieved by stratified charge combustion, for example, by directly injecting fuel into cylinders at the latter period of a compression stroke.
For the purpose of preventing environmental damage such as air pollution, an OBD (on-board diagnostic) system that detects or determines, and displays deterioration and/or failure of car parts for controlling exhaust gas has been introduced. The OBD system detects or determines malfunction of car parts for controlling exhaust gas, displays a warning to inform a driver of a malfunction when it occurs and stores a failure record thereof.
Especially, an apparatus has been developed for determining the occurrence of variation in air-fuel ratios among cylinders that is at a predetermined level or greater, in a multicylinder internal combustion engine. For example, see Japanese Laid-open Patent Publication No. 7-279732. In the apparatus of JP No. 7-279732A, rotational fluctuation of an output shaft of an internal combustion engine between the cylinders is detected during the lean-burn operation, and occurrence of the variation in air-fuel ratios is determined based on the detected rotational fluctuation. For example, rotational speed of the output shaft of the engine is detected per cylinder at a predetermined rotational angular range in a combustion stroke (for example, a period from a top dead center for compression to the time when the output shaft of the engine rotates by 30°), and the difference of the detected rotational speeds between the cylinders is used as the rotational fluctuation.
During the lean-burn operation of the internal combustion engine, the amount of fuel provided for combustion in the cylinders is small. Thus, misfires tend to occur when the air-fuel ratio is unnecessarily changed due to individual difference in the engine or variation with time. Therefore, the acceptable range of variation in the air-fuel ratio for operation of the engine is quite narrow. Thus, in an apparatus that determines occurrence of variation in air-fuel ratio through the comparison between the rotational fluctuations and a determination value, a scope of values the rotational fluctuation, which is a parameter for determining the occurrence of variation in air-fuel ratio, can take is also quite narrow. Then, a value that divides such a narrow scope into two areas, one being an area where the variations of air-fuel ratio occurs and another being an area where the variations of air-fuel ratio does not occur, which needs to be set as the determination value. When the determination of occurrence of the variations of air-fuel ratio is performed based on the determination value set for the narrow scope, accuracy of the determination is hard to improve. In this regard, there is room for improvement in the malfunction determination apparatus as described above.
An object of the present invention is to provide a diagnostic method and a diagnostic system for a multicylinder internal combustion engine that can determine occurrence of the malfunction pertaining to an air-fuel ratio with excellent accuracy.